Method and driver for programming phase change memory cell

ABSTRACT

In the method of programming a phase change memory cell, having a lower resistive state and a higher resistive state, to the lower resistive state, the memory cell is heated to first temperature. Subsequently, the memory cell is heated to second temperature, which is greater than the first temperature.

DOMESTIC PRIORITY INFORMATION

This is a divisional of U.S. application Ser. No. 10/845,065 filed May14, 2004; the contents of which are hereby incorporated by reference intheir entirety.

FOREIGN PRIORITY INFORMATION

The present invention claims priority under 35 U.S.C. 119 on KoreanApplication No. 10-2003-0056011 filed Aug. 13, 2003; the contents ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Solid state memory devices that use a structural phase-change materialas the data storage mechanism (referred to here simply as ‘phase-changememories’) offer significant advantages in both cost and performanceover conventional charge storage based memories. The phase-change memoryis made of an array of constituent cells where each cell has somestructural phase change material to store the cell's data. This materialmay be, for instance, a chalcogenide alloy that exhibits a reversiblestructural phase change from amorphous to crystalline. A small volume ofthe chalcogenide alloy is integrated into a circuit that allows the cellto act as a fast switching programmable resistor. This programmableresistor can exhibit greater than 40 times dynamic range of resistivitybetween a relatively crystalline phase (low resistivity) and arelatively amorphous phase (high resistivity). The data stored in thecell is read by measuring the cell's resistance. The chalcogenide alloycell is also non-volatile.

FIGS. 1A and 1B illustrate an example of a phase change memory cell. Asshown, the memory cell 10 includes a phase change material 12 disposedbetween a bottom electrode 14 and a top electrode 16. A bottom electrodecontact 18 provides for electrical contact between the bottom electrode14 and the phase change material 12. A transistor 20 selectivelysupplies a current to the memory cell 10 to selectively change the stateof the phase change material. FIG. 1A illustrates the state of the phasechange material 12 when reset (i.e., when in the amorphous state), andFIG. 1B illustrates the state of the phase change material 12 when set(i.e., in the crystalline state). As will be appreciated from FIG. 1A,the phase change material 12 is not completely changed to the amorphousstate when reset, and the phase change material 12 may not be completechanged to the crystalline state when set.

A conventional technique for programming a phase-change memory cell isto apply a rectangular pulse of current (having a constant magnitudethroughout the pulse) to the cell 10, at a voltage greater than aswitching threshold for the phase change material 12, which leaves thecell 10 in the reset state (the phase change material 12 is relativelyamorphous and has high resistivity). To change the state to a set state(the phase change material 12 is relatively crystalline and has lowresistivity), a rectangular lower current pulse, also at a voltagegreater than the switching threshold, is applied to the memory cell 10.The reset pulse has a higher magnitude of current than the set pulse sothat the temperature of the phase change material 12 is raised to anamorphizing temperature, before the phase change material 12 is rapidlycooled down or quenched by the very sharp decrease in current at thetrailing edge of the reset pulse; thereby leaving the phase changematerial 12 in the amorphous phase. To change into the crystallinephase, the phase change material 12 can be heated to a temperature,which is lower than the amorphizing temperature, using a rectangularcurrent pulse of smaller magnitude, and then rapidly cooled down again,this time leaving the phase change material 12 in the crystalline (lowresistance) phase. Here, the set pulse is considerably longer than thereset pulse.

It is also known to change the phase change material 12 into thecrystalline phase by heating the phase change material up to theamorphizing temperature and slowly reducing the applied current toreduce the temperature of the phase change material 12. As thetemperature of the phase change material 12 slowly decreases, the phasechange material 12 crystallizes. This method of setting the memory cellalso requires a significant amount of time.

SUMMARY OF THE INVENTION

In one embodiment of the method of programming a phase change memorycell according to the present invention, the memory cell has a lowerresistive state and a higher resistive state. The method includes firstheating the memory cell to first temperature, and subsequently, secondheating the memory cell to second temperature. In this embodiment, thesecond temperature is greater than the first temperature.

In one exemplary embodiment, the first temperature is an optimumnucleation temperature and the second temperature is the optimumcrystallization temperature.

A further exemplary embodiment includes heating, before the firstheating step, the memory cell to a third temperature where the thirdtemperature is greater than the second temperature. For example, in oneembodiment, the third temperature is great enough to change the memorycell towards the higher resistive state.

In another method of programming a phase change memory cell according tothe present invention, the memory cell is placed in the lower resistivestate by first applying a first current to the memory cell and thensecond applying a second current to the memory cell. In this embodiment,the second current is greater than the first current.

The method may further include placing the memory cell in the higherresistive state by applying a third current to the memory cell where thethird current is greater than the second current.

In one embodiment, the step of placing the memory cell in the lowerresistive state may apply a fourth current before application of thefirst current. In this embodiment, the fourth current is the samemagnitude as the third current.

According to embodiments of the present invention, the first current mayproduce a first temperature at the memory cell that is an optimumnucleation temperature, and the second current may produce a secondtemperature at the memory cell that is the optimum crystallizationtemperature.

An apparatus for programming a phase change memory cell having a higherresistive state and a lower resistive state, according to oneembodiment, includes a current application circuit configured to applycurrent to the memory cell and a controller controlling the currentapplication circuit. In one embodiment, the controller controls thecurrent application circuit to place the memory cell in the lowerresistive state by applying the first current to the memory cell andthen applying the second current to the memory cell

In another embodiment of the apparatus for programming a phase changememory cell having a lower crystalline state and a higher crystallinestate to the higher crystalline state, a heat application circuit isconfigured to heat the memory cell. This embodiment includes acontroller controlling the heat application circuit to place the memorycell in the higher crystalline state by heating the memory cell to thefirst temperature and then heating the memory cell to the secondtemperature.

The present invention provides numerous advantages. Included among themis a significantly reduced time to set the memory cell, for example,change the state of the phase change material of the memory cell to ahigher crystalline state.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawings,wherein like elements are represented by like reference numerals, whichare given by way of illustration only and thus are not limiting of thepresent invention and wherein:

FIG. 1A illustrates a conventional phase change memory cell showing thestate of the phase change material when the memory cell is reset (i.e.,when in the amorphous state);

FIG. 1B illustrates the conventional phase change memory cell showingthe state of the phase change material when the memory cell is set(i.e., in the crystalline state);

FIG. 2 illustrates a graph of temperature versus probability density fornucleation Pn and crystallization Pg of an example phase change materialsuch as chalcogenide material (Ge, Sb, Te);

FIGS. 3A and 3B illustrate a first example method for setting andresetting a memory cell according to the present invention;

FIG. 4 illustrates an example circuit diagram for a write driver circuitimplementing the example method of FIGS. 3A and 3B;

FIG. 5 illustrates an example of the signal selector in the write drivercircuit of FIG. 4.

FIG. 6 illustrates the waveforms of signals generated by the writedriver circuit of FIG. 4 during a set operation;

FIG. 7 illustrates the states of transistors in the write driver circuitof FIG. 4 during the set operation;

FIGS. 8A and 8B illustrate a second example method for setting andresetting a memory cell according to the present invention;

FIG. 9 illustrates an example circuit diagram for a write driver circuitimplementing the example method of FIGS. 8A and 8B;

FIG. 10 illustrates the waveforms of signals generated by the writedriver circuit of FIG. 9 during a set operation;

FIG. 11 illustrates the states of transistors in the write drivercircuit of FIG. 9 during the set operation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 illustrates a graph of temperature versus probability density fornucleation Pn and crystallization Pg of an example phase change materialsuch as chalcogenide material (Ge, Sb, Te). As shown, there exits anoptimal temperature PN in the 680–700° K at which nucleation occurs anda different, higher optimal temperature PG in the 700–800° K at whichcrystallization occurs for a phase change material such as chalcogenidematerial. In consideration of the above, the inventors discovered thatsetting a memory cell or crystallizing the phase change material couldbe achieved much more quickly if optimal nucleation takes place followedby optimal crystallization of the phase change material.

FIGS. 3A and 3B illustrate a first example method for setting andresetting a memory cell according to the present invention. For thepurposes of explanation only, this method will be described as appliedto the memory cell 10 of FIG. 1. FIG. 3A illustrates the current pulseswith respect to time that are applied to a memory cell 10 to reset andset the memory cell 10, and FIG. 3B illustrates the resultingtemperature of the phase change material 12 as a result of the appliedcurrent pulses.

As shown, a reset pulse as is conventional is applied to change thephase change material 12 to the amorphizing state and reset the memorycell 10. Namely, the current pulse heats the memory cell 10, and inparticular, the phase change material 12 to a melting temperature TM ofthe phase change material 12 and rapidly cools or quenches the phasechange material 12 by the very sharp decrease in current at the trailingedge of the reset pulse; thereby leaving the phase change material 12 inthe amorphous phase.

With respect to setting the memory cell 10, a set pulse having a firstcurrent ST1 for a first time and subsequently a second, higher currentST2 for a second time is applied. As shown, the second time is longerthan the first time, and the second and first times are longer than timeof the reset pulse. The first current ST1 heats memory cell 10, and inparticular, the phase change material 12, to the optimal nucleationtemperature PN, and the second current ST2 heats the memory cell 10 tothe optimal crystallization temperature PG. The times for applying thefirst and second currents ST1 and ST2 are a matter of design set basedon empirical study of the phase change material 12.

FIG. 4 illustrates an example circuit diagram for a write driver circuit600 implementing the example method described above with respect toFIGS. 3A and 3B. As shown, a controller 610 controls the operation of avoltage reduction circuit 620 and a current stage control circuit 640,and thus a current generating circuit 630 connected to the voltagereduction circuit 620 and the current stage control circuit 640. Thecurrent generating circuit 630 applies current to the memory cell 10.The controller 610 receives an externally applied write data pulse DTD_P(e.g., by asserting certain pins of a memory chip employing the writedriver circuit 600) and a mode control signal SQM, and causes thecurrent generating circuit 630 to apply the reset and set pulses inaccordance with the external signals.

Specifically, the controller 610 includes a control signal generator 612that generates a plurality of control signals: a first control signalP1, a second control signal P2, a reset control signal P_RESET and a setcontrol signal P_SET based on the write data pulse DTD_P and the modecontrol signal SQM. When the mode control signal SQM is logic high, thenthe control signal generator 612 is set to a first mode. In the firstmode, the control signal generator 612 generates the first controlsignal P1, the second control signal P2, the reset control signalP_RESET and the set control signal P_SET such that the current pulsesaccording to the embodiment of FIGS. 3A–3B may be generated. When themode control signal SQM is logic low, then the control signal generator612 is set to a second mode. In the second mode, the control signalgenerator 612 generates the first control signal P1, the second controlsignal P2, the reset control signal P_RESET and the set control signalP_SET such that the current pulses according a conventional method ofsetting and resetting a memory cell may be generated.

When the write data pulse DTD_P of logic high is received during thefirst mode, this triggers the control signal generator 612 to generatethe first control signal P1, the second control signal P2, the resetcontrol signal P_RESET and the set control signal P_SET as shown in FIG.6, described in detail below.

As shown in FIG. 4, a signal selector 614 of the controller 610 controlsthe output of the reset control signal P_RESET as a reset signal RESETand the set control signal P_SET as a set signal SET based on anexternally applied write data signal WDATA. FIG. 5 illustrates anexample of the signal selector 614. As shown, a first multiplexer 6142selectively outputs one of the reset control signal P_RESET and a logiclow value based on the state of the write data signal WDATA, and asecond multiplexer 6144 selectively outputs one of the set controlsignal P_SET and a logic low value based on the state of the write datasignal WDATA. Namely, when the write data signal WDATA is logic high,the first multiplexer 6142 outputs the reset control signal P_RESET asthe reset signal RESET and the second multiplexer 6144 outputs logiclow. When the write data signal WDATA is logic low, the firstmultiplexer 6142 outputs logic low as the reset signal RESET and thesecond multiplexer 6144 outputs the set control signal P_SET as the setsignal SET.

Returning to FIG. 4, the current stage control circuit 640 includes afirst NAND gate 641 receiving the first control signal P1 and the setsignal SET. The output of the first NAND gate 641 controls the operationof a first PMOS transistor TR1. The first PMOS transistor TR1 isconnected between a power supply voltage VCC and a control node N1.Accordingly, the first PMOS transistor TR1 is turned on and supplies thepower supply voltage VCC to the control node N1 when the first controlsignal P1 and the set signal SET are both logic high (i.e., the outputof the NAND gate 641 is logic low); otherwise, the first PMOS transistorTR1 is turned off.

A second PMOS transistor TR2 is also connected between the power supplyvoltage VCC and the control node N1, and has its gate also connected tothe control node N1.

The voltage reduction circuit 620 includes first and second NMOStransistors TR3 and TR4 connected in parallel between the control nodeN1 and ground VSS. The gate of the first NMOS transistor TR3 receivesthe reset signal RESET. Accordingly, when the reset signal RESET islogic high, the voltage at the control node N1 is pulled down becausethe first NMOS transistor TR3 turns on. Otherwise, when the reset signalRESET is logic low, the voltage at the control node N1 is substantiallyunaffected by the first NMOS transistor TR3.

The gate of the second NMOS transistor TR4 receives the output of aninverter 623, which receives the output of a second NAND gate 621. Thesecond NAND gate 621 receives the set signal SET and the second controlsignal P2. Accordingly, when the second control signal P2 and the setsignal SET are logic high: the output of the second NAND gate 621 islogic low, the output of the inverter 623 is logic high, and the secondNMOS transistor TR4 is turned on. This operation pulls down the voltageat the control node N1. When the second control signal P2 and the setsignal SET are not both logic high: the second NAND gate 621 outputslogic high, the inverter 623 outputs logic low, the second NMOStransistor TR4 is turned off, and the voltage at the control node N1 issubstantially unaffected by the second NMOS transistor TR4.

As further shown in FIG. 4, the current generating circuit 630 includesa third PMOS transistor TR5 connected between the power supply voltageVCC and the memory cell 10. The gate of the third PMOS transistor TR5 iscontrolled by the voltage at the control node N1.

Next the operation of the write driver circuit 600 illustrated in FIG. 4will be described in detail with respect to FIG. 4 and FIGS. 6 and 7.FIG. 6 illustrates the waveforms of signals generated by the writedriver circuit 600 during a set operation, and FIG. 7 illustrates thestates of the first PMOS transistor TR1 and the first and second NMOStransistors TR3 and TR4 during the set operation.

As shown in FIG. 6, when the mode control signal SQM is logic high and alogic high write data pulse DTD_P is received, the control signalgenerator 612 generates the reset control signal P_RESET, the setcontrol signal P_SET, the first control signal P1 and the second controlsignal P2 at time t1. As shown, the first control signal P1, the secondcontrol signal P2, the reset control signal P-RESET and the set controlsignal P_SET have fixed amplitudes and durations. The durations andamplitudes of these control signals are a matter of design andestablished based on empirical study of the phase change material 12.The first control signal P1 has a duration commensurate with a desiredduration of the first current ST1 and the second control signal P2 has aduration commensurate with a desired duration of the first and secondcurrents ST1 and ST2 (see FIG. 3A). Furthermore, the set control signalP_SET has a duration at least as long as the second control signal P2,and the reset control signal P-RESET has a duration commensurate with adesired duration of a reset current pulse such as shown in FIG. 3A. Thecontrol signal generator 612 behaves in the manner described aboveregardless of whether a set or reset operation is to take place.

When a logic low write data signal WDATA is received indicating to setthe memory cell 10, the signal selector 614 outputs the logic low valueas the reset signal RESET, and outputs the set control signal P_SET asthe set signal SET. Accordingly, for the duration (t2–t1) of the firstcontrol signal P1, the first control signal P1 and the set signal SETare logic high and the first NAND gate 641 turns on the first PMOStransistor TR1. This operation results in the application of the supplyvoltage VCC to the control node N1. During this same time period, thesecond control signal P2 and the set signal are logic high and thesecond NAND gate 621 and inverter 623 turn on the second NMOS transistorTR4. This operation results in the voltage at the control node N1 beingpulled down. Also, the logic low reset signal RESET is applied to thefirst NMOS transistor TR3 such that the first NMOS transistor TR3 isoff.

The resulting voltage at the control node N1, turns on the third PMOStransistor TR5 a sufficient amount that the first current ST1 (see FIG.3A) is applied to the memory cell 10. As shown in FIG. 6, for the timeperiod t1 to t2, which is the duration of the first control signal P1,this first current ST1 is applied.

When the first control signal P1 transitions to logic low at time t2,the set signal SET is still logic high. As such, the first NAND gate 641turns off the first PMOS transistor TR1. The supply voltage VCC is nolonger supplied to the control node N1 by the first PMOS transistor TR1.However, the second control signal P2 and the set signal SET are stilllogic high, and the second NAND gate 621 and inverter 623 keep thesecond NMOS transistor TR4 on. As a result, the voltage at the controlnode N1 is pulled down, and the second PMOS transistor TR2 turns on.Current flows from the supply voltage VCC through the second PMOS andNMOS transistors TR2 and TR4 to ground VSS. Because the second PMOStransistor TR2 has a size greater than the first PMOS transistor TR1,the conductive or current carrying capacity of the second PMOStransistor TR2 is greater than that of the first PMOS transistor TR1.Consequently, the control node N1 is pulled down to a lower voltage thanduring time t1 to t2. The third PMOS transistor TR5 becomes moreconductive, and the second current ST2 (see FIG. 3A) is supplied to thememory cell 10 as shown in FIG. 6.

When the second control signal P2 transitions to logic low, the secondNAND gate 621 and the inverter 623 turn off the second NMOS transistorTR4 such that the control node N1 reaches a high voltage state. Namely,the second PMOS transistor TR2 supplies the power supply voltage to thecontrol node N1, which effectively turns off the third PMOS transistorTR5 and ends the supply of current to the memory cell 10 as shown inFIG. 6.

Accordingly, the write driver circuit 600 supplies current as shown inFIG. 3A to the memory cell 10 to set the memory cell 10 by applying heatof the temperatures depicted in FIG. 3B. This sets the memory cell 10 ina significantly reduced time.

Next, the reset operation of the write driver circuit 600 will bedescribed. In this operation, the write data signal WDATA is logic highindicating to reset the memory cell 10. As a result, the signal selector614 generates a logic low set signal SET and outputs the reset controlsignal P_RESET as the reset signal RESET.

Because the set signal SET is logic low, the NAND gate 641 turns off thefirst PMOS transistor TR1. Also, the second NAND gate 621 and theinverter 623 turn off the second NMOS transistor TR4.

The logic high reset signal RESET turns on the first NMOS transistorTR3, which pulls down the voltage at the control node N1. The secondPMOS transistor TR2 also turns on and current flows from the supplyvoltage VCC through the second PMOS and first NMOS transistors TR2 andTR3 to ground VSS. As the control node N1 is pulled down, the third PMOStransistor TR5 becomes more conductive, and a reset current as shown inFIG. 3A is generated.

The size of the first NMOS transistor TR3 is greater than the size ofthe second NMOS transistor TR4 such that the first NMOS transistor TR3has a greater capacity to pull down the control node N1 and cause thethird PMOS transistor TR5 to supply a reset current that is greater thanthe second current ST2. As mentioned above, the size of the first PMOStransistor TR1 is less than the size of the second PMOS transistor TR2,and the size of the second PMOS transistor TR2 is less than or equal tothe size of the third PMOS transistor TR5.

FIGS. 8A and 8B illustrate a second example method for setting andresetting a memory cell according to the present invention. For thepurposes of explanation only, this method will be described as appliedto the memory cell 10 of FIG. 1. FIG. 8A illustrates the current pulseswith respect to time that are applied to a memory cell 10 to reset andset the memory cell 10, and FIG. 8B illustrates the resultingtemperature of the phase change material 12 as a result of the appliedcurrent pulses.

As shown, a reset pulse as is conventional is applied to change thephase change material 12 to the amorphizing state and reset the memorycell 10. Namely, the current pulse heats the memory cell 10, and inparticular, the phase change material 12 to a melting temperature TM ofthe phase change material 12 and rapidly cools or quenches the phasechange material 12 by the very sharp decrease in current at the trailingedge of the reset pulse; thereby leaving the phase change material 12 inthe amorphous phase.

With respect to setting the memory cell 10, a set pulse having aninitial current ST0 is applied for an initial time followed byapplication of the currents ST1 and ST2 for first and second times asdescribed above with respect to FIG. 3A. The initial current ST0 may bethe same as the reset pulse in magnitude and duration. Accordingly, theinitial time is less than the first time, which is less than the secondtime. The application of the initial current ST0 changes the phasechange material 12 to the amorphous state. However, unlike the resetoperation, after applying the initial current ST0, the first current ST1is applied to heat memory cell 10, and in particular, the phase changematerial 12, at the optimal nucleation temperature PN. Then, the secondcurrent ST2 heats the memory cell 10 to the optimal crystallizationtemperature PG. The times for applying the currents ST0, ST1 and ST2 area matter of design set based on empirical study of the phase changematerial 12.

FIG. 9 illustrates an example circuit diagram for a write driver circuit800 implementing the example method described above with respect toFIGS. 8A and 8B. As shown, a controller 810 controls the operation of avoltage reduction circuit 820 and a current stage control circuit 840,and thus the current generating circuit 830 connected to the voltagereduction circuit 820 and the current stage control circuit 840. Thecurrent generating circuit 830 applies current to the memory cell 10.The controller 810 receives an externally applied write data pulse DTD_P(e.g., by asserting certain pins of a memory chip employing the writedriver circuit 800) and a mode control signal SQM, and causes thecurrent generating circuit 830 to apply the reset and set pulses inaccordance with the external signals.

Specifically, the controller 810 includes a control signal generator 812that generates a plurality of control signals: a first control signalP1, a second control signal P2, a third control signal P3, a resetcontrol signal P_RESET and a set control signal P_SET based on the writedata pulse DTD_P and the mode control signal SQM. When the mode controlsignal SQM is logic high, then the control signal generator 812 is setto a first mode. In the first mode, the control signal generator 812generates the first control signal P1, the second control signal P2, thethird control signal P3, the reset control signal P_RESET and the setcontrol signal P_SET such that the current pulses according to theembodiment of FIG. 8A may be generated. When the mode control signal SQMis logic low, then the control signal generator 812 is set to a secondmode. In the second mode, the control signal generator 812 generates thefirst control signal P1, the second control signal P2, the third controlsignal P3, the reset control signal P_RESET and the set control signalP_SET such that the current pulses according a conventional method ofsetting and resetting a memory cell may be generated.

When the write data pulse DTD_P of logic high is received during thefirst mode, this triggers the control signal generator 812 to generatethe first control signal P1, the second control signal P2, the thirdcontrol signals P3, the reset control signal P_RESET and the set controlsignal P_SET as shown in FIG. 10 described in detail below.

As shown in FIG. 9, a signal selector 814 of the controller 810 controlsthe output of the reset control signal P_RESET as a reset signal RESETand the set control signal P_SET as a set signal SET based on anexternally applied write data signal WDATA. Namely, the signal selector810 has the same structure and operates in the same manner as the signalselector 614 described above with respect to FIG. 5.

The current stage control circuit 840 includes a first NAND gate 841receiving the second control signal P2, the set signal SET and aninverted version of the third control signal P3. The third controlsignal P3 is inverted by a first inverter 843 before being supplied tothe first NAND gate 841. The output of the first NAND gate 841 controlsthe operation of a first PMOS transistor TR1. The first PMOS transistorTR1 is connected between a power supply voltage VCC and a control nodeN1. Accordingly, the first PMOS transistor TR1 is turned on and suppliesthe power supply voltage VCC to the control node N1 when the secondcontrol signal P2 and the set signal SET are both logic high and thethird control signal P3 is logic low (i.e., the output of the NAND gate641 is logic low); otherwise, the first PMOS transistor TR1 is turnedoff.

The current stage control circuit 840 also includes a current reductioncontrol signal generating circuit 842. The current reduction controlsignal generating circuit 842 includes a second inverter 845 invertingthe second control signal P2 and a second NAND gate 847 receiving outputof the second inverter 845, the first control signal P1 and the setsignal SET. A third inverter 849 inverts the output of the second NANDgate 849. An NOR gate 851 performs a NOR operation on the output of thethird inverter 849 and the reset signal RESET, and a fourth inverter 853inverts the output of the NOR gate 851 to produce the current reductioncontrol signal. As will be appreciated, the current reduction controlsignal has a logic low state only with the output of the third inverter849 and the reset signal RESET are logic low; otherwise, the currentreduction control signal has a logic high state. Furthermore, the outputfrom the third inverter 849 is only logic high when the first controlsignal P1 is logic high, the second control signal P2 is logic low andthe set signal SET is logic high; otherwise, the output of the thirdinverter 849 is logic low.

A second PMOS transistor TR2 is also connected between the power supplyvoltage VCC and the control node N1, and has its gate also connected tothe control node N1.

The voltage reduction circuit 820 includes first and second NMOStransistors TR3 and TR4 connected in parallel between the control nodeN1 and ground VSS. The gate of the first NMOS transistor TR3 receivesthe output of an OR gate 825, which ORs the reset signal RESET and thecurrent reduction control signal. Accordingly, when the reset signalRESET is logic high or the current reduction control signal is logichigh, the voltage at the control node N1 is pulled down because thefirst NMOS transistor TR3 turns on. Otherwise, when the reset signalRESET is logic low and the current reduction control signal is logiclow, the voltage at the control node N1 is substantially unaffected bythe first NMOS transistor TR3.

The gate of the second NMOS transistor TR4 receives the output of afifth inverter 823, which receives the output of a third NAND gate 821.The third NAND gate 821 receives the set signal SET and the secondcontrol signal P2. Accordingly, when the second control signal P2 andthe set signal SET are logic high: the output of the third NAND gate 821is logic low, the output of the inverter 823 is logic high, and thesecond NMOS transistor TR4 is turned on. This operation pulls down thevoltage at the control node N1. When the second control signal P2 andthe set signal SET are not both logic high: the third NAND gate 821outputs logic high, the inverter 823 outputs logic low, the second NMOStransistor TR4 is turned off, and the voltage at the control node N1 issubstantially unaffected by the second NMOS transistor TR4.

As further shown in FIG. 9, the current generating circuit 830 includesa third PMOS transistor TR5 connected between the power supply voltageand the memory cell 10. The gate of the third PMOS transistor TR5 iscontrolled by the voltage at the control node N1.

Next the operation of the write driver circuit 800 illustrated in FIG. 9will be described in detail with respect to FIG. 9 and FIGS. 10 and 11.FIG. 10 illustrates the waveforms of signals generated by the writedriver circuit 800 during a set operation, and FIG. 11 illustrates thestates of the first PMOS transistor TR1 and the first and second NMOStransistors TR3 and TR4 during the set operation.

As shown in FIG. 10, when the mode control signal SQM is logic high andthe write data pulse DTD_P is received, the control signal generator 812generates the reset control signal P_RESET, the set control signalP_SET, and the first control signal P1 at time t1. The control signalgenerator 812 generates a second control signal at a subsequent time t2and generates the third control signal P3 at a further subsequent timet3. As shown, the first control signal P1, the second control signal P2,the third control signal P3, the reset control signal P-RESET and theset control signal P_SET have fixed amplitudes and durations. Thedurations and amplitudes of these control signals are a matter of designand established based on empirical study of the phase change material12. The first control signal P1 has a duration commensurate with adesired duration over which the currents ST0, ST1 and ST2 are applied asshown in FIG. 8A. The second control signal P2 has a durationcommensurate with a desired duration over which the currents ST1 and ST2are applied as shown in FIG. 8A, and the third control signal has aduration commensurate with a desired duration over which the current ST2is applied as shown in FIG. 8A. As will be described in more detailbelow, the time t2–t1 between when the first and second control signalsP1 and P2 are generated defines the duration that the initial currentST0 is applied; and the duration t3–t2 defines the duration that thefirst current ST1 is applied.

Furthermore, the set control signal P_SET has a duration at least aslong as the first control signal P1, and the reset control signalP_RESET has a duration commensurate with a desired duration of a resetcurrent pulse such as shown in FIG. 8A. The control signal generator 812behaves in the manner described above regardless of whether a set orreset operation is to take place.

When a logic low write data signal WDATA is received indicating to setthe memory cell 10, the signal selector 814 outputs the logic low valueas the reset signal RESET, and outputs the set control signal P_SET asthe set signal SET. Accordingly, for the duration (t2–t1), only thefirst control signal P1 and the set signal SET are logic high. Asdescribed above, because the second control signal P2 is logic low, thefirst NAND gate 841 generates a logic high and turns off the first PMOStransistor TR1.

Also during the t1 to t2 time period, the logic low second controlsignal P2 and logic high set signal SET result in the third NAND gate821 output a logic high and the inverter 823 outputting a logic low.This turns the second NMOS transistor TR4 off.

During this same time period, the current reduction control signalgenerator 842 generates a logic high current reduction control signalbecause, as described above, the first control signal P1 is logic high,the second control signal P2 is logic low, the set signal SET is logichigh and the reset signal RESET is logic low. As a result, the OR gate825 generates a logic high signal and turns on the first NMOS transistorTR3, which pulls down the voltage at the control node N1. The secondPMOS transistor TR2 also turns on and current flows from the supplyvoltage VCC through the second PMOS and first NMOS transistors TR2 andTR3 to ground VSS. As the control node N1 is pulled down, the third PMOStransistor TR5 becomes conductive, and an initial current ST0 as shownin FIG. 8A is generated.

When the second control signal P2 transitions to logic high at time t2,the conditions for generating a logic high current reduction controlsignal no longer exist, and the current reduction control signaltransitions to logic low. The reset signal RESET is logic low as well,and the first NMOS transistor TR3 turns off.

However, as described above, a logic high second control signal P2,logic low third control signal P3 and logic high set signal SET resultin the first NAND gate 841 turning on the first PMOS transistor TR1.This operation results in the application of the supply voltage VCC tothe control node N1. During this same time period, the second controlsignal P2 and the set signal are logic high, and the third NAND gate 821and inverter 823 turn on the second NMOS transistor TR4. This operationresults in the voltage at the control node N 1 being pulled down.

The resulting voltage at the control node N1, turns on the third PMOStransistor TR5 a sufficient amount that the first current ST1 (see FIG.8A) is applied to the memory cell 10. As shown in FIG. 10, for the timeperiod t2 to t3, which is the duration between the second control signalP2 being generated and the third control signal P3 being generated, thisfirst current ST1 is applied.

When the third control signal P3 transitions to logic high at time t3,the current reduction control signal remains unchanged such that thefirst NMOS transistor TR3 remains off. Also, the on state of the secondNMOS transistor TR3 is unaffected by the transition of the third controlsignal P3. However, the transition of the third control signal P3 doesresult in the first PMOS transistor TR1 being turned off. This operationprevents the supply of the power supply voltage VCC to the control nodeN 1 via the first PMOS transistor TR1. Because the second NMOStransistor TR4 continues to pull down the control node N1, the secondPMOS transistor TR2 turns on and a current flows to ground VSS throughthe second PMOS and first NMOS transistors TR2 and TR4. Because thesecond PMOS transistor TR2 has a size greater than the first PMOStransistor TR1, the conductive or current carrying capacity of thesecond PMOS transistor TR2 is greater than that of the first PMOStransistor TR1. Consequently, the control node N1 is pulled down to alower voltage than during time t2 to t3. The third PMOS transistor TR5becomes more conductive, and the second current ST2 (see FIG. 8A) issupplied to the memory cell 10 as shown in FIG. 10.

When the first, second and third control signals P1, P2 and P3transition to logic low, the first PMOS transistor TR1, first NMOStransistor TR3 and the second NMOS transistor TR4 turn off. The secondPMOS transistor TR2 supplies the power supply voltage to the controlnode N1, which effectively turns off the third PMOS transistor TR5 andends the supply of current to the memory cell 10 as shown in FIG. 10.

Accordingly, the write driver circuit 800 supplies current as shown inFIG. 8A to the memory cell 10 to set the memory cell 10 by applying heatof the temperatures depicted in FIG. 8B. This sets the memory cell 10 ina significantly reduced time.

Next, the reset operation of the write driver circuit 800 will bedescribed. In this operation, the write data signal WDATA is logic highindicating to reset the memory cell 10. As a result, the signal selector814 generates a logic low set signal SET and outputs the reset controlsignal P_RESET as the reset signal RESET.

Because the set signal SET is logic low, the NAND gate 841 turns off thefirst PMOS transistor TR1. Also, the second NAND gate 821 and theinverter 823 turn off the second NMOS transistor TR4.

The logic high reset signal RESET turns on the first NMOS transistor TR3via the OR gate 825, and this pulls down the voltage at the control nodeN1. The second PMOS transistor TR2 also turns on and current flows fromthe supply voltage VCC through the second PMOS and first NMOStransistors TR2 and TR3 to ground VSS. As the control node N1 is pulleddown, the third PMOS transistor TR5 becomes more conductive, and a resetcurrent as shown in FIG. 8A is generated.

The size of the first NMOS transistor TR3 is greater than the size ofthe second NMOS transistor TR4 such that the first NMOS transistor TR3has a greater capacity to pull down the control node N1 and cause thethird PMOS transistor TR5 to supply a reset current or initial currentST3 that is greater than the second current ST2. As mentioned above, thesize of the first PMOS transistor TR1 is less than the size of thesecond PMOS transistor TR2, and the size of the second PMOS transistorTR2 is less than or equal to the size of the third PMOS transistor TR5.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the present invention.

1. A method of programming a phase change memory cell, having a lowerresistive state and a higher resistive state, to the lower resistivestate, comprising: first heating the memory cell to first temperature;and second heating the memory cell to second temperature following thefirst heating step, the second temperature being greater than the firsttemperature.
 2. The method of claim 1, wherein the first temperature isless than an optimum crystallization temperature for the memory cell. 3.The method of claim 2, wherein the first temperature is an optimumnucleation temperature.
 4. The method of claim 3, wherein the optimumnucleation temperature falls within a range of 680–700° K.
 5. The methodof claim 3, wherein the second temperature is the optimumcrystallization temperature.
 6. The method of claim 5, wherein theoptimum nucleation temperature falls within a range of 680–700° K andthe optimum crystallization temperature falls within a range of 700–800°K.
 7. The method of claim 5, further comprising: third heating, beforethe first heating step, the memory cell to a third temperature, thethird temperature being greater than the second temperature.
 8. Themethod of claim 7, wherein the third temperature is great enough tochange the memory cell towards the higher resistive state.
 9. The methodof claim 2, wherein the second temperature is the optimumcrystallization temperature.
 10. The method of claim 9, wherein theoptimum crystallization temperature falls within a range of 700–800° K.11. The method of claim 2, further comprising: third heating, before thefirst heating step, the memory cell to a third temperature, the thirdtemperature being greater than the second temperature.
 12. The method ofclaim 11, wherein the third temperature is great enough to change thememory cell towards the higher resistive state.
 13. The method of claim1, wherein the second temperature is greater than an optimum nucleationtemperature.
 14. The method of claim 13, wherein the second temperatureis an optimum crystallization temperature.
 15. The method of claim 14,further comprising: third heating, before the first heating step, thememory cell to a third temperature, the third temperature being greaterthan the second temperature.
 16. The method of claim 15, wherein thethird temperature is great enough to change the memory cell towards thehigher resistive state.
 17. The method of claim 13, further comprising:third heating, before the first heating step, the memory cell to a thirdtemperature, the third temperature being greater than the secondtemperature.
 18. The method of claim 17, wherein the third temperatureis great enough to change the memory cell towards the lower crystallinestate.
 19. The method of claim 1, wherein the first and secondtemperature are less than a melting temperature of a phase changematerial forming part of the memory cell.
 20. The method of claim 1,wherein the first heating step heats the memory cell for a first time;and the second heating step heats the memory cell for a second timegreater than the first time.
 21. The method of claim 20, furthercomprising: third heating, before the first heating step, the memorycell to a third temperature for a third time, the third temperaturebeing greater than the second temperature and the third time being lessthan the first time.
 22. A driver for a phase change memory cell havinga lower crystalline state and a higher crystalline state to the highercrystalline state, comprising: a heat application circuit configured toheat the memory cell; and a controller controlling the heat applicationcircuit to place the memory cell in the higher crystalline state byfirst heating the memory cell to a first temperature and second heatingthe memory cell to a second temperature following heating of the memorycell the first temperature, the second temperature being greater thanthe first temperature.